Nameserver redundancy

How many nameservers answer for the domain. Two or more, in different places, keep the site resolvable when one of them fails.

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Nameservers are the authoritative DNS servers for your domain: every visitor's resolver asks them where the website and the mail servers are. If all of them are unreachable, the domain disappears — the site, the mail and every API on it — as soon as cached answers expire, even though the web server itself is fine.

How SiteScan checks

The scan reads the NS records of the registrable domain. Two or more nameservers are green; a single one is a warning, because it is a single point of failure for everything on the domain. This is the one real fault in the infrastructure half of this group. The Domain registration group may show the nameservers as listed by the registry, which should match.

How to improve it

• Use at least two nameservers, ideally on different networks. Managed DNS providers give you several by default, usually anycast and spread across continents.
• If you run your own, put the secondary on a different provider or data centre, and make sure zone transfers keep it in sync.
• Check that the NS records in the zone match the delegation at the registrar; stale entries pointing at an old provider cause intermittent failures.
• For extra resilience, some teams use two independent DNS providers serving the same zone.

Related: DNSSEC and domain registrar.